This invention relates to measuring the permeability to air movement across a plant leaf.
The measurement of permeability to air movement across a leaf has potential importance in plant science because that air moves almost entirely through the adjustable valves, or stomata, on the upper and lower leaf surfaces. Air permeability can thus serve as an indication of how open the stomata are. This is important because only when the stomata are open can the leaf take up carbon dioxide for photosynthesis and lose water in transpiration. Stomata close due to water deficit, low light, disease and other variables, and there has always been a need for a visible indicator of that closure event in plant management, education, and research.
There are three areas of relevant prior art:
a. Various leaf "porometers" which measure air permeability have been developed for plant research purposes (reviewed by Meidner,1981,In Jarvis and Mansfield,Stomatal Physiology,Cambridge:Cambridge University Press,pp33-49; and Weyers and Meidner,1990,Methods in Stomatal Research,UK:Longman Group). All apply a differential air pressure across the leaf by clamping a sealed chamber on one or both sides and measuring the resulting flow rate. This technique is generally practical only for plants which have stomata on both sides of the leaf, and this is a limitation of the present instrument as well. However, a large majority of plant species, and virtually all crop plants, have stomata on both sides and hence can be measured with this technique. PA1 b. Diffusion porometers (such as that commercially available from Li-Cor, Inc., Lincoln, Neb.) have been developed which measure the permeability of the leaf surfaces to water vapor. Since vapor flow, like air flow, is largely through the stomata, this measurement can be related to stomatal opening or "stomatal resistance". All these instruments suffer from errors due to an unknown vapor density within the leaf, small temperature differences between the leaf and air ,and the possibility of influencing stomatal aperture if the instrument stays in contact with the leaf for more than a short time. All these instruments are also bulky and too expensive (more than $6,000 for Li-Cor) to be available to farmers, gardeners, high schools and colleges. PA1 c. Devices for measuring the air permeability of sheet materials have been extensively developed for the paper industry. None of these are obviously adaptable to plant leaves in regard to the requirements of portability, making a non-destructive seal on plant leaf surfaces, or quantifying any lateral leaks which may occur between the gasket and leaf surface.
Most of these experimental air flow porometers have been designed for laboratory work and are completely unsuitable for a portable, rapid, non-destructive measurement in the field. Of those designed for field work (Alvim,1965,A new type of porometer for measuring stomatal opening and its use in irrigation studies,UNESCO Arid Zone Research,25:325-329; Bierhuizen et al.,1965,A poromter for laboratory and field operation,J.Exp.Bot.,16:182-191; Weatherly,1966,A porometer for use in the field,New Phytol.,65:376-387; Milburn,1979,An ideal viscous flow porometer,J.Exp.Bot.,30:1021-1034) all suffer from the difficulty of establishing a temporary but absolute seal between the applied pressure chamber and an often rough leaf surface. Without a dependable seal there is no way of knowing how much of the measured flow is actually through the leaf. All these instruments also use a relatively high driving pressure (at least 60 inches of water). This requires potentially damaging clamping pressure and may also influence stomatal aperture in various ways (Meidner, 1981). And none of these instruments is, or is capable of being, self contained in a compact unit held and operated with one hand.